Macroscopic and microscopic phenomena of electro-discharge machined steel surfaces: An experimental investigation

Journal o/Mechanical Working Technology, 15 (1987) 335-356

335

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

M A C R O S C O P I C A N D MICROSCOPIC P H E N O M E N A OF ELECTRO-DISCHARGE MACHINED STEEL SURFACES: AN EXPERIMENTAL INVESTIGATION

A.G. MAMALIS, G.C. VOSNIAKOS, N.M. VAXEVANIDIS

Department of Mechanical Engineering, National Technical Universityo/Athens, Athens (Greece) and J. PROHASZKA

Department o/Materials Sciences, Technical University o/Budapest, Budapest (Hungary) (Received February 2, 1987; accepted in revised form March 4, 1987)

Industrial summary Electro-discharge machining is a thermal process with a complex metal removal mechanism, involving the formation of a plasma channel between the tool and workpiece electrodes, melting and evaporation action and shock waves, resulting in phase changes, tensile residual stresses, cracking and metallurgical transformation. These properties determine the operational behaviour of machined parts and are included in the term "surface integrity". Experimental results pertaining to the physico-chemical changes occurring during electro-discharge machining of steel (structural, medium-carbon and alloyed steels) surfaces are reported in this paper. Surface morphology and finish are assessed and correlated with overall process parameters and metal removal rates. Metallurgical transformations and new structures, surface damage in the form of cracking and "white layer" formation, microhardness variations and residual stress patterns are quantitatively and qualitatively examined and discussed. The results confirm the inherent complexity of the process, as well as the assumptions that must be made in order to achieve acceptable predictions of the mechanical and physico-chemical characteristics of the machined surfaces.

Notation

acl d¢

[t hc i n Ra

Rmax t

average crack length crater diameter total pulse frequency crater depth pulse current coefficient centre-line-average surface roughness peak-to-valley surface roughness pulse duration

0378-3804/87/$03.50

© 1987 Elsevier Science Publishers B.V.

336 U

We wlt ~r

discharge voltage volumetric metal removal rate metal removal per pulse pulse energy white-layer thickness residual stress

Introduction

To achieve electro-discharge machining (EDM) by the preferential erosion of the work electrode, a succession of discrete discharge pulses of particular nominal, though actually varying, characteristics is applied between the tool and the workpiece, representing the anode and the cathode, immersed in a dielectric fluid. Stability of operating conditions is usually secured by a servocontrolled mechanism, so that the cutting tool impresses its complementary shape on the workpiece with a small "over-cut". EDM is considered especially suitable for machining complex contours, for high accuracy and for materials that are not amenable to conventional removal methods. Complex physical processes take place during spark-erosion, both locally and temporarily overlapping. A complete, clear and scientifically admissible theory of EDM has not yet been established; three main approaches have been developed [ 1 ]. (1) Attribution of metal removal to the extremely strong electric field produced near the electrode surfaces and the generation of a shock on the positive ions of the metal lattice, exceeding the cohesion forces in it. This mechanism, however, can account only for the action of very short pulses. (2) For longer pulses the basic mechanism of erosion is that of melting and evaporation [ 2,3 ]. (3) The best explanation of ED phenomena is offered by the thermo-electric theory, as establishedby extensive experimental studies [ 4-6 ]. Three stages can be distinguished: (i) Ionization and arc formation at a localized area between the electrodes, following the application of a voltage exceeding the breakdown voltage. (ii) The occurrence of the main discharge as an electron avalanche striking the anode; low electrical resistance in the discharge channel, hydraulic restriction of the dielectric and the magnetic pinch effect establish high current densities. The cathode is struck by ions and is heated less rapidly than the anode. (iii) Local melting and evaporation follow and material is removed from the site of the discharge by explosion occurring after the cessation of the electrical discharge. The current density decreases with increasing discharge duration, the discharge tending to become an arc. De-ionization of the

337 plasma channel occurs after the completion of the whole cycle and a new cycle can start at the site of the closest electrode distance. The scope of this work is to examine, comprehensively, the influence of the intense thermal nature of phenomena involved in EDM upon the main features of steel surfaces produced by this process. Information is given on microgeometry, surface topography, metallurgical structure in the surface and subsurface layers, residual stress state, microhardness and plastic deformation; their dependence on the main parameters of the process is examined. These properties determine the operational behaviour of machined parts and are included in the term "surface integrity" [7 ].

Experimental details

Equipment and test materials The experimental work was carried out on an EMT110/AGIE industrial machine. Electrolytic copper of a rectangular work area 40 × 22 mm 2 was used for tool electrode (of positive polarity). Lateral flushing with a pressure of 0.5 bar was used in a hydrocarbon dielectric. Rectangular pulses were produced from the machine generator, the test conditions being: pulse current i= 6, 12, 18, 22 and 26 A; pulse duration t= 50, 75, 100, 300 and 500 tts; open voltage 100 V. The test materials used were low-carbon steel St37, medium carbon steel C45 and alloyed steel 100Cr6. Square plates of surface dimensions 40 × 40 mm 2 and of 4 m m thickness were used as the test specimens. The pulse energy, usually quoted in the literature as being an important machining parameter, is given by

We=ftu(t) i(t) d t = u i t

(1)

Ignition delay is ignored and u was kept constant at 30 V.

Measurements (i) Optical microscopy: The electro-discharge machined specimens were sectioned transversely and prepared under the standard procedure for metallographic observation. Etching was performed by immersing the specimens in 3% Nital, in Fry's microetching solution (5 g CuC12, 40 ml HC1, 25 ml C2H5OH, 30 ml H20) and in Schrader's reagent (1 g Picric acid, 10 ml HC1, 10 ml HNO3, 80 ml C2HsOH). The depths of various layers on the transverse section were measured on a U N I M E T metallographic microscope. (ii) Scanning electron microscopy: Sample strips 5 × 5 m m 2 were cut from electro-discharge machined specimens and their surfaces were examined on a Cambridge Stereoscan electron microscope, at an illumination angle of 30 °.

338 (iii) Microhardness testing: The microhardness tests were run on a Leitz machine with a Vickers indenter. The specimens used were prepared using standard metallographic methods. (iv) Surface roughness measurements: Surface profiles as well as surface roughness values Ra and Rmaxwere obtained from a "Talysurf" recorder (Taylor Hobson) and a "Dr. FSrster" stylus instrument. The cut-off was set at 0.25 m m and the roughness values were the average of at least 10 measurements per specimen.

(v) Residual stress measurements: Examination of residual stresses in electro-discharge machined specimens has been most often determined using destructive methods, i.e. by removing successive surface layers and measuring the resulting deflection. In the present work the X-ray diffraction method, which is more advantageous than destructive methods [ 7 ], was used to determine residual stress distributions; measurements were performed on a Hitachi "Strainflex" stress analyzer MSF-2M and PSF-2M (characteristic ray: Cr K~, tube voltage 30 kV). Thermal treatment for stress relief preceded machining. A precision balance, micrometer and stop-watch were used for determining relative wear and metal removal rates. (vi) X-Rayphase analysis: A Philips 800 mW cobalt X-ray tube with an iron filter was used for X-ray phase analysis of typical C45 and 100Cr6 spark-eroded specimens. Samples were scanned using K~¢I) radiation of 1.78892 A wavelength, from 0 = 25 - 65 ° with the help of an HZG3 RFT-type diffractometer. Preparation of specimens consisted of degreasing and cleaning them ultrasonically. Results and discussion

Metal removal and surface topography The peculiarity of an electro-discharge machined surface is determined by various factors associated with a complex erosion mechanism. Randomly overlapping craters of dimensions varying with pulse energy, see Fig. 1, cover the machined surface, their sizes and positions reflecting the stochastic nature of the process. Typical surface structures of St37 specimens are shown in Figs. 2 (a) and (b) ; the thermal nature of the metal removal p h e n o m e n o n is evident. The difference in crater dimensions is due to the difference in nominal pulse energy whilst a more "turbulent"-like appearance of the molten and re-solidified surface material results from rougher machining conditions. Surface cracking was observed for high pulse energies, both by electron and optical microscopy. These cracks extend underneath the surface to depths that depend on the pulse en-

339

(a)

S t 3 7 ; We:23Z, mJ

, 300;urn ,

Fig. 1. Micrographsshowing(a) singleand (b) overlappingcratersofelectro-dischargemachined steel surfaces. ergy, see Fig. 3. Arcing, generated under conditions of low current density in the plasma channel, was found to enhance surface-crack formation, see Fig. 3 (b). The absence of cracks was confirmed by SEM supporting optical metallography for pulse energies as low as 36 mJ. Spherical particles and "pock marks" are shown in Fig. 4: the former are re-solidified metal droplets attached to the crater sides, whilst the latter look like remnants of bubble evolution. In fact, local melting and evaporation of the workpiece material occur during and after an electrical discharge and its expulsion often takes place with gas evolution [ 8,9 ]. This violent material removal from each individual crater results in an overall volumetric metal removal rate, which together with relative electrode wear,

340

I

50 p m

St37;

!

St 37:

50 ~m

J

We= 36 rnJ

Fig. 2. Scanningelectron micrographs,showingthe structures of typical electro-dischargemachinedsteelsurfaces. is an important quantity, its value depending on the process parameters, the machining conditions and the physical properties of the anode and the cathode. The variation of relative wear and metal removal rate with respect to pulse duration and current follows general well-known patterns [ 6]. Both metal removal and wear increase with pulse current; relative wear apparently decreases with pulse duration, but for the metal removal rate an optimum duration seems to exist, below which metal removal increases with pulse duration, this relation being reversed above the optimum level, see Fig. 5.

341

Moterial

:

100Cr6

l,

10 p m

,,I

Fig. 3. Micrographs showing surface cracks in electro-dischargemachined steel plates by (a) scanning electron microscopy (b) optical microscopy. The shape and volume of a crater during the E D M process is not only directly related to the surface roughness, but can also be used for an approximate calculation of the metal removal rate. A simple approximate approach to relating average crater dimensions estimated from optical microscopy measurements to overall metal removal rate has been made. In general, the metal removal rate 'dw, can be expressed as:

Vw=nft Vwc

(2)

where Vw¢ is the metal removal per pulse, ft is the total pulse frequency and n is a coefficient to account for: (i) the inclusion of inactive pulses in ft (e.g. open circuits, arcings etc); (ii) the ignoring of any ignition delay; and (iii)

342

J

C45;

50 .,urn

I

We= 162 mJ

Fig. 4. Scanning electron micrographs showing spherical particles and "pock marks" in electrodischarge machined steel surfaces. errors in the statistical estimation of average crater dimensions from optical microscopy. The experimental results, assuming a spherical crater, give an average coefficient n=0.299 +_17.6% (see Table 1 ). The influence of pulse current and duration upon the average crater dimensions, i.e. crater diameter dc and crater depth he, are expressed by the following empirical regression equations: For St37: ~ d~= 11.275 io.494to.32~ h~= 3.937 io.2~6to.27~ For 100Cr6: de = 15.760 io.523 to.225 (3) he-- 9.465 i°1~6t°195

343

~150 i=26 A (100C r6 ) \ o.

E E .:~.~ 100

o o

4~

o

O

otCj__ ° . . . . + ! / . i = 1 8 A ( 5 t 37 ) /

+ 50

lifO--

\ i = 1 2 A ( S t 37)

@

"6 i

i

I

100

200

300

Pulse d u r a t i o n ,

t

I

1

400

500

(~.lsec)

Fig. 5. Variation in volumetric metal removal rate with pulse duration for electro-discharge machined steel plates.

Pulse current seems to be more significant for diameter variations, whilst pulse duration is more important for depth variations. Thus, there is an increase in crater dimensions with pulse energy, because of the larger penetration of melting isothermals [3 ]. These observations are physically related to the corresponding alterations of the heat source, i.e. the plasma channel (current density and channel diameter), the thermal properties of the materials being of major importance, and with the assumption that there are sufficient forces available for material ejection. Examination of the characteristic crater diameter-to-depth ratio shows that it increases with pulse energy for low and high energies, whilst for moderate energies it is approximately constant, see Fig. 6 and Table 1. This implies that above a particular energy level, which is different for different workpiece materials, the crater expands mainly in width and not in depth, following a nonproportional widening of the plasma channel with discharge energy [ 11 ].

Surface roughness Centre-line average, Ra, and maximum peak-to-valley, Rma~, surface roughness measurements of electro-discharge machined surfaces were taken to provide quantitative evaluation of the influence of EDM parameters on surface finish*. *The shortcomings of surface finish assessment through conventional stylus-recorders are generally known; stylus radius, cut-off and measuring length are blamed for this non-reliability. Moreover, characteristic features of electro-discharge machined surfaces such as cracks, "pockmarks", and re-solidified droplets, described previously, emphasize the problem. However, "talysurf" measurements are very simple to make and in the present investigation were confirmed and complemented by optical and electron microscopy observations.

Specimen No.

8 3 13 14A 15 20 6 2 21 22

17 14B 9 16 10

Material

St37 St37 8t37 St37 St37 St37 St37 St37 St37 St37

100Cr6 100Cr6 100Cr6 100Cr6 100Cr6

Experimental results

TABLE 1

39 78 180 234 390

9 18 36 78 41 108 162 180 198 234

Pulse energy, We ( m J )

5.50 6.50 7.40 7.50 8.15

3.55 3.90 5.70 6.50 5.85 6.60 7.25 7.80 7.50 7.60

Measured

R, (]Lm)

8.47 8.04 10.39 11.54 12.64

4.09 4.92 6.55 7.24 6.66 8.53 9.53 9.58 10.10 10.91

Calculated

Surface roughness

27.0 28.5 -

8.4 20.0 23.5 26.0 27.5 29.4

Measured

Rm., (~m)

104 121 52 120 115

21 19 52 124 75 54 105 53 119 126

Measured

1006.5 360.0 128.9 366.5 379.6

88.5 71.9 220.7 415.6 361.1 181.6 270.9 173.7 356.2 506.6

Calculated

M e t a l removal rate Vw (mm3/mm)

0.10 0.34 0.39 0.33 0.30

0.24 0.26 0.23 0.29 0.21 0.29 0.39 0.30 0.33 0.25

240 205 235 300 385

100 115 180 240 200 250 290 300 325 375

Diameter dc ( # m )

36 35 47 50 53

18 22 28 30 28 36 40 40 42 45

D e p t h hc (pro)

Crater dimensions

6.67 5.86 5.00 6.00 7.26

5.56 5.23 6.43 8.00 7.14 6.94 7.25 7.50 7.74 8.33

dc hc

du-

345 lO £

~

i St 3 7

8

o

•,-, u 6 V

100Cr6 E cI

2 u

0

I

I

I

100

200

300

Pulse

energy,W

400

e (mJ)

Fig. 6. Variation of crater diameter-to-depth ratio with pulse energy for electro-dischargemachined steel plates. The surface roughness increases with increasing nominal pulse energy, see Fig. 7. The relationship between surface-roughness (/~m) and pulse energy We (mJ) can be approximated by least-square equations of the 3rd degree: For St37:

t R,,

= 4.42+ 50.58 W~e/3

Rmax =15.06+234.30 We~3 (4) For l00Cr6: Ra -- 5.83+ 20.21 W~/3 On the other hand, increasing pulse duration or pulse current results in increasing R,, the variation being more pronounced for small values of current and duration (R, is approximately 30% more affected by changes in current than it is by changes in duration), as obtained from the appropriate regression equations (3), expressing the diameter and the depth of the craters for both St37 and 100Cr6 specimens. Pulse current has a greater effect upon the expansion of the melting isothermal. In general, measured values of R, for St37 electro-discharge machined plates slightly overestimate - - by about 5% m the corresponding values for 100Cr6 plates for the same machining conditions, see Fig. 7; the thermal conductivity is greater for St37 specimens, which exhibit a higher metal removal rate as compared with 100Cr6 specimens, the difference in metal rate being approximately 3-10%, whilst C45 specimens are machined even faster. It is also worth mentioning that for an electro-discharge machined surface to be twice as smooth as another, requires roughly a 7-to-8 fold reduction in the metal removal rate, see Fig. 8. This is a common feature of manufacturing processes; especially in EDM, a high metal removal rate not only results in poor surface finish, but is also associated with intensification of surface damage such as crack generation, etc. To overcome this problem, fine cutting conditions may be imposed at the final stage of processing. k

346

~ 30 E

R m a x ( S t 37 ) / o / °

E -j

\

%

~/

o/

•

y ...v-.

cr"

/ Ro ( 5 t 3 7 ) ~'O _...... + --'t"- "

.+.....-~ V

v 0

20

m~

E a:

u3

/

o~"

Ra ( 1 0 0 C r 6 )

c-

I

.c

4

O

10

o

@

u

a

tD

I

J

100 Purse

1

200

300

energy,

We ( r n J )

4O0

Fig. 7. Variation of surface roughness with pulse energy for electro-discharge machined steel plates. 9 E ="

%

8

Materiet

St 37

¢,f

m-

u1

7

c-

+-

6

t3n

o ~-

0

5

(9

o

,+-

4

0

I

I

I

L

l

i

20

40

60

80

100

120

140

Metal removaL r a t e , ~/w(mm3/ m i n ) Fig. 8. Variation of surface roughness with volumetric metal removal rate for electro-discharge machined steel plates.

The improvement of surface finish with machining time was examined by machining four different St37 specimens to different depths of cut under exactly the same conditions. For a pulse energy of 180 mJ a decrease in mean surface roughness of about 1 #m per mm of cut was obtained, as the initial biased discharge at the peaks of the machined surface gradually spread homogeneously to produce a smoother surface. Considering the ideal case of spherical adjacent craters having dimensions as measured (by optical microscopy) for the specimens examined, an upper bound to the surface roughness may be calculated from the simple expression 4

hc

Ra =-~--~ 4(he~de)2+ 1

(5)

347

(a) S t 3 7 ;

We=180mJ

300pro

;

Fig. 9. Micrographs showingthe formation of a "white layer" on electro-discharge machined steel surfaces. Calculated values from eqn. (5) are given in Table 1 along with the experimentally measured values; the former overestimate the latter by about 26% on average, a coefficient accounted for by the assumptions made and by any experimental errors incurred.

Microstructure and damage Numerous investigations have been carried out to reveal microstructure and phase changes, deformation patterns and damage markings on surfaces of var-

348

Fig. 10. Micrographshowingthe microstructureof the "white layer" on an electro-dischargemachined steel plate. ious materials electro-eroded under various machining conditions. Different zones have been defined, characterized by the presence of the above-mentioned phenomena [ 1,8,9]. Molten material resulting from the discharge action undergoes extremely high cooling rates while in contact with the base material, and consequently metastable conditions are expected. Crack formation and traces of plastic deformation are also worthy of examination. For this purpose, optical microscopy was used in the present work. The commonly described "white layer" was found along the surface of St37, 100Cr6 and C45 specimens for all of the pulse energies applied. Etching of the specimens with Nital reagent left this area characteristically white. In most cases the white layer was discontinuous, whilst in some it appeared to be continuous and of a constant thickness, see Fig. 9; signs of gas-evolution pores and craters are noticeable in these figures. The surface configuration evident when using SEM could not, of course, become so evident at this stage. A needle-like structure of white layer material is shown in Fig. 10; the increased carbon content which is expected in this layer (mainly as a result of dielectric cracking and the severe machining conditions) has been reported to play a significant role [ 8 ]. The general pattern of white layer probably consists of a fine dispersion of carbides in an austenitic matrix or a ledeburitic structure as far as steel is concerned [ 12]. The effect of lateral flushing was the building-up of resolidified material (white layer) at the outer edge of the machined surface (Ucontour), as well as the greater thickness of the "white layer" downstream {always with reference to dielectric flow). X-rays were taken to reveal the phase changes during processing. Fig.

349

FeQ (110)

M a t e r i a l : C 45 (i) Before EDM

F

Fe,,, (211) Fea (220)

I 65

Fee

I 60 e (°) (ii)

I 55

I 50

I

45

(200)

I

!

40

35

I

30

25

4

After EDM Feb (100)

FeQ (220)

_ I

65

Fea (211)

Fea (200)

Cu1(311)\\ I

60

1

55

~'-.~Cu

(111)

cu1(220) l ~ ~ , I

50

,

I

45

I

40

I

35

I

30

I

25

e (°)

(o)

Fig. 11 (a). X-ray determination of phase changes in the EDM of steel plates: C 45.

11 (a) (i) shows the phase lines of a C45 specimen in its original state (before processing) and Fig. l l ( a ) ( i i ) the corresponding phase lines in the sparkeroded state. By comparison of these X-ray diagrams, it becomes evident that during the EDM process a new f.c.c, phase is formed on the eroded surface, mainly due to the copper of the tool electrode; from the measurements it was found that the copper concentration on the surface was 1.6 wt%. Figure 11 (b) is related to the 100Cr6 steel. In the original state (before processing), this material contains Fe~, Fey and Fe3C phases, see Fig. 11 (b) ( i ); ferrite, austenite and cementite are common in this material because of its hardened state. After spark-erosion, an additional iron-chrome phase Fe3Cr (112) appears in the material, see Fig. 11 (b) (ii). Judging from the linebreadth decrease of Fe~, a complex Fe/Cr reaction may occur simultaneously with surface cementite formation (by depleting the carbon content of the Fe~, i.e. the ferrite phase). Note that a new line FeCr(211) with a b.c.c, ordered

350

M a t e r i a l : 100 C r 6

Fe3Cr (112) ]'7 , ,

( i ) Before EDM

Fea(1OO)~l 1

.-.--I__~

65

60 e (o) ~

I

I

I

55

50

45

40

35

30

25

Fe3Cr (112) (ii) After E D M

Fe~ (100) ~ Fe~(21t)

~y(311)

Fay(220) ~

i

I

"1

50

45

_ L _ _ L

55

e (°)

FeCr (211) _ ,_ ,\ /

Fe-~C (130)

~

\ \

' I

40

" J.

35

~ey ~,'~)A) ~ Iv V.,) 3~3

25

--,,--

(b) Fig. II.(b) X-ray determination of phase changes in the E D M of steelplates:100Cr6.

structure is observed. Note also that the copper lines, as in the case of C45, see Fig. 11 (a), are not apparent in this X-ray plot, but they may exist together with the lines of the F%: to clarify this effect more experimental work is needed, which is now under-way. Surface damage of electro-discharge machined surfaces appears also as crack formation, associated with the development of high thermal stresses exceeding the fracture strength of the material, as well as with plastic deformation. Cracks, usually referred to as microcracks, are initiated at the eroded surface, propagate through the "white layer" and sometimes, as grain-boundary cracks, they extend into layers of the undeformed material underneath the surface, see Fig. 12. The brittleness of the "white layer" due to increased carbon content and alloying effects from the tool electrode, is favourable for crack generation. The length and number of cracks depend on the pulse energy and also on the thermal properties of the workpiece material. Average crack length was found to be greater for 100Cr6 specimens than for St37 specimens, for the same machining conditions. For pulse energy as low as 1 8 m J there was no cracking. Most cracks observed ran normally to the eroded

351

!

150 pm

100Cr6; We=180 mJ

i

I

Fig. 12. Micrograph showing crack formation and propagation across the thickness of an electrodischarge machined steel plate. 6o

60

:3-

E :3v

d

wlt(lOOCr6 ) e

/

U 0

e

i

40 I

E

24-' 20

/+"

+

:t-

~

~

~-o--

v ''j ~act(St

oc[(100Cr6 )

u 0 L..

20

37)

u 0 Ib >

r-

~

o

L

I

I

1

100

200

300

400

0 500

Pulse energy, We (m J) Fig. 13. Variation of white layer thickness (wit) and average crack length (acl) with pulse energy for electro-discharge machined steel plate.

surface and nearly all of them did not exceed the white-layer boundary. Some cracks extending from the bottom of the crater in a radial direction were also observed and should be related to tangential residual stresses, whilst others were directed parallel to the surface. Surprisingly, many cracks appeared at the intersection of the horizontal and vertical sides of the machined U-contours. White layer thickness and crack lengths observed were averaged for each specimen; the results are plotted against pulse energy in Fig. 13. It is obvious that cracks are developed inside the white layer and that both white layer thickness and average crack length are greater for the 100Cr6 specimens; the thermal properties of the material make the main contribution to this difference. The following empirical equations were derived relating white layer

352

Fig. 14. Micropgraph showing the formation of a maztensitic zone in an electro-discharge ma-

chined steelplate.

Fig. 15. Micrograph showing the heat-affected zone in an electro-dischargemachined steelplate.

thickness, wlt, and average crack length, acl, to pulse duration and current for St37: wit = 3.438 io.19s to.ao6 acl = 0.628 io.sgs t0.301

(6)

These equations indicate the importance of pulse duration as far as the white layer thickness is concerned and the greater influence of pulse current on crack average length. However, no sensible relationship between surface damage

353 800

•~

MaterialSt

37

"r" 600

r"

"~ 400 .£ 0 i_

u

:7

200

0

O"~ O~

O~

We= 234 mJ

I

I

I

I

50

100

150

200

Depth

below surface

250

(lJm)

Fig. 16. Showing the variation in microhardness over the thickness of electro-discharge machined steel plates.

% 5oo z

400

Material

A.,~.e~., ~

100 Cr 6

A~ ' ~ ' ~ A

E

~ 300 L,

~ 200

Material

St 37

"o 100 ® rr

0

I

i

I

I

100

200

300

401

Pulse

e n e r g y , W e (m J)

Fig. 17. Variation of surface residual stresses with pulse energy for electro-discharge machined steel plates.

characteristics and R, or Rmax could be derived as has been proposed earlier [ 12 ]. From Fig. 13 it can be concluded that conditions for minimizing surface damage are far from coinciding with those corresponding to high erosion rates. Surface cracks can be finally removed by the finishing process but the cracks running inside the material must be studied in detail, because they can affect the fatigue strength, especially when the machined components are exposed to high stresses and temperatures. Heat-affected zones have been observed for both St37 and 100Cr6 plates but they differ in structure. Under the white layer of the latter a layer of martensite and retained austenite was observed, see Fig. 14; its thickness varied between 35 and 50/~m approximately. Another type of heat-affected zone was revealed

354 by etching the worked surface with Fry's reagent, see Fig. 15. The width of this zone has been found to vary according to the white layer mean thickness variation, ranging between 6 and 17 #m. This zone was observed for St37 specimens and its presence can be associated with dendritic growth and the absence of preferential direction; note the marked difference in grain size between this zone and the undeformed material. Small grain sizes are, as known, generally favourable for the strength of the material. Microhardness measurements Microhardness studies are important for identifying yield strength changes, structure alterations such as the formation of martensite, work hardening or softening of surface layers, etc [ 7 ]. Light loads make it possible to record the local hardness values only, avoiding interference from layers underneath the surface and allowing the determination of hardness gradients. The variation of microhardness on the surface layers of a St37 specimen is shown in Fig. 16; the surface hardness rises to nearly four times that of the undeformed material. Martensitic structures as well as increased carbon content saturated from dielectric pyrolysis normally account for such differences. For C45 and mainly for 100Cr6 specimens, it was found that the "transition" layer below the surface results in comparatively higher microhardness values than those of the surface layer, which may be explained by the change in chemical composition due to spark-erosion. It is believed that higher pulse energy and the resulting increased penetration of the isothermals causes the increased martensite formation and the deeper carbon diffusion, so that the high-hardness zone is widened [7,8]. For St37 specimens this zone was found to extend to a thickness 2 to 5 times greater than their corresponding peak-to-valley maximum surface roughness. An analogy to this extremely steep hardness gradient is that of a temperature gradient ranging from ambient to material boiling temperature over a depth of a few hundredths of a mm. Distribution of residual stress Static and dynamic strength, stress corrosion resistance, chemical resistance, magnetic properties and possible deformation after removal of material are among the main properties strongly influenced by the presence of residual stresses [ 7 ]. On the other hand, high tensile residual stresses can be a continuous danger for crack propagation under tensile working stresses or even useful under compressive stressing. Residual stresses measured for both St37 and 100Cr6 specimens were tensile; their maximum value, immediately under or near to the surface, was comparable to the strength of the material, at least for St37, see Fig. 17. Steep temperature gradients due to the rapid thermal cycle at the surface and thermal contraction of re-solidified material on the base material, including plastic deformation, result in the generation of tensile residual stress [ 10 ]. Residual

355 stress patterns may be altered by the occurrence of metallurgical transformations involving volumetric changes; martensitic transformation from austenite is known to take place with a simultaneous increase in specific volume of about 3%. From Fig. 17 it may be concluded that a non-strictly monotonic variation of stress magnitude with pulse energy may be expected, a trend also characterising other aspects of EDM, e.g. material removal rate and expansion of the diameter of the plasma channel [ 11 ]. The negative slope of the curves of Fig. 17 above a particular energy level may be associated with the increase, of about 1.4-2 times, of the length of the cracks penetrating the "white layer" or even deeper. These cracks may result from residual stress values overcoming the UTS of the material, their formation constituting a kind of stress relief. The energy lower bound for stress relief depends on the workpiece material, as indicated in Fig. 16; it is known, however, that higher thermal conductivity reduces the penetration of isothermals and makes the temperature gradient steeper [ 10]. Conclusions

Summarising the main observations of the present experimental work on EDM of St37, C45 and 100Cr6 steel plates, the following conclusions may be drawn: (a) The peculiarities of the electro-discharge machined surface arising from the complexity of the metal removal mechanism have been revealed using optical and scanning electron microscopy. Random, overlapping, craters cover the machined surface, which shows a "turbulent" appearance; an increase in crater dimensions with pulse energy has been observed. (b) Surface roughness is related to the cubic root of pulse energy. Better thermal conductivity of the workpiece material enhances the metal removal rate but is to the detriment of the surface finish. Increasing machining time improves the surface roughness. An upper bound to the surface roughness can be estimated from measurements of the crater dimensions. (c) Microstructure changes and different heat-affected zones, according to the material examined, have been observed using metallographic etching and X-ray phase analysis; the results reflect the complex phase changes occurring during EDM. "White layer" and crack formation are associated with the development of high thermal stresses exceeding the fracture strength of the material as well as with plastic deformation and are determined quantitatively by the use of regression equations; their dimensional dependence on pulse energy is clearly shown. (d) Microhardness changes confirm microstructural changes, whilst X-ray diffraction enables the accurate determination of residual stresses of tensile

356 n a t u r e a n d o f a m a g n i t u d e o f t e n a p p r o a c h i n g t h e s t r e n g t h o f t h e material; stress r e l a x a t i o n usually o c c u r s for h i g h e r pulse energies. Acknowledgements We are grateful to M r s Z h o u g Rui Mei o f t h e Beijing R e s e a r c h I n s t i t u t e o f M e c h a n i c a l a n d E l e c t r i c a l T e c h n o l o g y , M M B I , Beijing, C h i n a for t h e residual stress m e a s u r e m e n t s , to t h e T e c h n i c a l S t a f f o f t h e L a b o r a t o r y o f M a n u f a c t u r ing T e c h n o l o g y o f t h e N a t i o n a l T e c h n i c a l U n i v e r s i t y o f A t h e n s for h e l p i n g w i t h t h e e x p e r i m e n t a l w o r k a n d to M r s R o u l a T s o p e l a for t y p i n g our manuscript.

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